• All seven Central European tree species tested showed a similar ability to stabilise heavy metals in contaminated soil.
• Tree species with low nutrient needs keep excess metals out of their stems and foliage, making the production of uncontaminated timber possible.
• Selecting suitable tree species could bring abandoned land back into productive use while also safely confining the soil pollutants.

For centuries, artisanal and industrial activities have left many soils polluted with excessive levels of heavy metals such as zinc, cadmium, copper and lead, posing a threat to the environment and human health. Cleaning up polluted sites is often difficult and expensive, especially when larger areas are affected. As a result, many sites need to be managed in a way that prevents pollutants from washing away into groundwater or neighbouring ecosystems.

One promising solution is phytostabilisation— using plants to reduce the spread of pollutants into the environment. Trees are particularly well suited because of their long lifespans and extensive root systems. To find out which tree species to select for this purpose, researchers from the Swiss Federal Research Institute WSL and ETH Zurich compared the stabilisation potential of seven Central European tree species.

The researchers grew the trees for five years in soils containing heavy metal-concentrations similar to those found in contaminated sites. They measured metal concentrations in the roots, the stems and the leaves and compared them with trees growing in uncontaminated soil. The results have been published as the cover story of the journal Plants.

The selected tree species represent two contrasting growth strategies (see Box). Four “acquisitive” species, including grey alder (Alnus incana), aspen (Populus tremula), osier willow (Salix viminalis) and silver birch (Betula pendula), require large amounts of nutrients. On the other hand, three “conservative” species, namely Norway spruce (Picea abies), European beech (Fagus sylvatica) and sycamore maple (Acer pseudoplatanus), have lower nutrient requirements.

As a result, the wood of conservative species contained no more heavy metals than wood from trees grown in uncontaminated soil. “We were surprised by how clear-cut the distinction was between the two growth strategies”, says Madeleine Günthardt-Goerg, first author of the study. "This gives us practical guidance on which tree species to choose for polluted sites".

Once abandoned, polluted sites are often spontaneously reclaimed by forest trees. Selecting suitable species with conservative growth strategies could help confine contaminants in the soil, prevent metals from recirculating through falling and decomposing leaves, while also yielding uncontaminated wood products.

“Adapted forest management may allow us to bring polluted sites back into the economic cycle in a cost-effective way”, says Pierre Vollenweider, researcher at WSL and last author of the study. “The next step would be to test this approach under real conditions at polluted sites.”

Box: What are growth strategies?

Trees can be broadly categorised into two ecological or growth strategies.

1. Acquisitive species need higher amounts of nutrients, tend to grow faster and complete their life cycle more rapidly.
2. Conservative species require smaller quantities of nutrients, sequester more carbon and generally have longer life spans.